Reductant addition in exhaust system comprising NOx-absorbent

ABSTRACT

An exhaust system for a vehicular lean-burn internal combustion engine comprises a NO x -absorbent, a reductant injector ( 78 ) disposed upstream of the NO x -absorbent and means ( 50 ), when in use, for controlling reductant addition, wherein the reductant addition control means supplies reductant to the NO x -absorbent at all vehicle speeds in a duty cycle at a rate which is predetermined to correlate with a desired NO x  conversion at the average duty cycle speed of the vehicle.

The present invention relates to an exhaust system for a lean-burninternal combustion engine comprising a NO_(x)-absorbent and, inparticular, to a method of controlling reductant addition into theexhaust system for the purpose of regenerating the NO_(x)-absorbent andreducing NO to N₂.

An exhaust system for a lean-burn internal combustion engine such as adiesel engine or a lean-burn gasoline engine comprising aNO_(x)-absorbent is known from, for example, EP 0560991.

As used herein, a “NO_(x)-trap” is a catalyst comprising aNO_(x)-absorbent and a catalyst for oxidising NO to NO₂. NO_(x)-trapsare also known as “lean NO_(x) traps” or “LNC”.

A typical NO_(x)-trap formulation includes a catalytic oxidationcomponent, such as Pt, a NO_(x)-absorbent, such as compounds of alkalimetals e.g. potassium and/or caesium; compounds of alkaline earthmetals, such as barium or strontium; or compounds of rare-earth metals,typically lanthanum and/or yttrium; and a reduction catalyst, e.g.rhodium. One mechanism commonly given for NO_(x)-storage during leanengine operation for this formulation is that, in a first step, the NOreacts with oxygen on active oxidation sites on the Pt to form NO₂. Thesecond step involves adsorption of the NO₂ by the storage material inthe form of an inorganic nitrate.

Whilst the inorganic NO_(x)-storage component is typically present as anoxide, it is understood that in the presence of air or exhaust gascontaining CO₂ and H₂O it may also be in the form of the carbonate orpossibly the hydroxide.

When the engine runs intermittently under enriched conditions or atelevated temperatures, the nitrate species become thermodynamicallyunstable and decompose, producing NO or NO₂. Under richer conditions,these NO_(x) species are reduced by carbon monoxide, hydrogen andhydrocarbons to N₂, which can take place over the reduction catalyst.

An object of an exhaust system comprising a NO_(x)-trap is to improvethe economy of the engine whilst meeting the relevant emissionsstandard, e.g. Euro N.

Systems to control reductant addition for the purpose of regenerating aNO_(x)-trap and reducing desorbed NO are known, but tend to require verycomplicated control regimes involving multiple sensor inputs andprocessors to run complex algorithms. As a result, such systems are veryexpensive.

EP-B-0341832 (incorporated herein by reference) describes a process forcombusting particulate matter (PM) in diesel exhaust gas, which methodcomprising oxidising NO in the exhaust gas to NO₂ on a catalyst,filtering the PM from the exhaust gas and combusting the filtered PM inthe NO₂ at up to 400° C. Such a system is available from Johnson Mattheyand is marketed as the CRT®.

We have investigated methods of regenerating NO_(x)-absorbents and wehave discovered that it is possible to meet a relevant emissionstandard, such as Euro IV, with an exhaust system comprising aNO_(x)-absorbent without the need for complex equipment such asalgorithm-programmed processors and a network of sensor inputs. Such adiscovery has particular application to the retrofit market.

According to a first aspect of the invention, there is provided anexhaust system for a vehicular lean-burn internal combustion enginecomprising a NO_(x)-absorbent, a reductant injector disposed upstream ofthe NO_(x)-absorbent and means, when in use, for controlling reductantaddition, wherein the reductant addition control means suppliesreductant to the NO_(x)-absorbent at all vehicle speeds in a duty cycleat a rate which is predetermined to correlate with a desired NO_(x)conversion at the average duty cycle speed of the vehicle.

The invention of the first aspect has particular application to theretrofit market for vehicles of a limited duty cycle such as buses orrefuse trucks. The idea is to determine what rate of reductant injectionis required to reduce a chosen quantity of NO_(x), e.g. 90%, in aNO_(x)-absorbent at the average duty cycle speed. For example, when theNO_(x)-absorbent is a component of a NO_(x)-trap, the system controllercan be arranged, when in use, to generate a continuous tempo andquantity of hydrocarbon (HC) fuel injection e.g. injection at 2 secondsevery minute. The system controller can also be arranged to provideoccasional relatively long rich HC fuel pulses to ensure that theNO_(x)-trap is substantially completely regenerated, followed by themore frequent sequence of shorter enrichment pulses to maintain thestoring capability of the NO_(x)-trap. The exact detail of the injectionstrategy depends on the vehicle and its duty cycle.

At speeds higher than the average duty cycle speed, there would be moreNO and a greater mass airflow and so NO_(x) conversion overall wouldfall off, because there would be insufficient reductant. However,because higher speed would be less likely e.g. in city centre buses, thesystem can meet NO emission standards over an entire drive cycle withoutincreasing fuel penalty; equally where the vehicle speed drops below theaverage duty cycle speed, HC can be emitted, but on average over a dutycycle the system can meet the emission standard for HC. The correlationof the rate of HC injection to average duty cycle speed can be tailoredto the particular application, e.g. buses in Manchester (UK) city centrewould be expected to encounter different duty cycles to those in London(UK) city centre.

In one embodiment of the first aspect, an oxidation catalyst is disposedbetween the reductant injector and the NO_(x)-absorbent for increasingthe temperature of the NO_(x)-trap for regeneration and/or to removeoxygen from the exhaust gas to ensure a rich exhaust gas forregeneration of the NO_(x)-absorbent.

In a particular arrangement, the NO₁-trap and systems for deliveringreductant described herein are disposed downstream of the arrangementdescribed in EP-B-0341832, mentioned hereinabove. That is, a catalystfor oxidising NO to NO₂ is followed by an optionally catalysed filterthen a reductant injector followed by the NO_(x)-absorbent.

In one embodiment, the NO_(x)-absorbent for use in the invention is acomponent of a NO_(x)-trap.

Unless otherwise described, the catalysts for use in the presentinvention are coated on high surface area substrate monoliths made frommetal or ceramic or silicon carbide, e.g. cordierite, materials. Acommon arrangement is a honeycomb, flow-through monolith structure offrom 100-600 cells per square inch (cpsi) such as 300-400 cpsi(15.5-93.0 cells cm², e.g. 46.5-62.0 cells cm²).

The internal combustion engine can be a diesel or lean-burn gasolineengine, such as a gasoline direct injection engine. The diesel enginecan be a light-duty engine or a heavy-duty engine, as defined by therelevant legislation.

A method of reducing NO_(x) in the exhaust gas of a vehicular lean-burninternal combustion engine according to a second aspect of the inventioncomprises absorbing NO_(x) from the exhaust gas in a NO_(x) absorbent,contacting the NO_(x) absorbent with a reductant to regenerate theNO_(x)-absorbent at all vehicle speeds in a duty cycle, and reducingNO_(x) to N₂, wherein a rate of reductant injection correlates with adesired NO_(x) conversion at the average duty cycle speed.

In order that the present invention may be more fully understood,embodiments thereof will now be described with reference to theaccompanying drawings, in which:

FIG. 1 shows a schematic system according to the first aspect of theinvention;

FIG. 2 is a schematic graph plotting quantity of fuel against timeshowing a fuel injection strategy for use in the system of FIG. 1;

FIG. 3 is a schematic of a working embodiment of the invention;

FIG. 4 is a graph showing the upstream Air/Fuel Ratio (AFR) as afunction of road speed in the embodiment of FIG. 3;

FIG. 5 is a graph showing NO_(x) measurements at the idle condition forthe embodiment of FIG. 3;

FIG. 6 is a graph showing the corresponding system temperatures at theidle condition for the trace shown in FIG. 5;

FIG. 7 is a graph showing NO_(x) measurements at 40 mph for theembodiment of FIG. 3;

FIG. 8 is a graph showing the corresponding temperature measurements at40 mph for the trace shown in FIG. 7; and

FIG. 9 is a graph showing the NO_(x) conversion as a function of roadspeed for the system of FIG. 3.

In the system 50 depicted in FIG. 1, 52 is a conditional systemcontroller (CSC), 54 is a master switch, 56 is an alternator, 58 is ablocking capacitor, 60 is a thermocouple, 62 is an injection controller(ICU), 64 is a fuel pump, 66 is a valve, 68 is a fuel injector and 70 isa positive power line. The CSC 52 is a switch providing power to the ICU62 if the master power switch 54 is on, the engine is running asdetermined by an AC ripple from the alternator 56 present after a DCblocking capacitor 58 and the output of a suitably placed thermocouple60 to detect the exhaust system is above a minimum pre-determinedtemperature for reduction of NO_(x) on a suitable NO_(x)-trap. Themaster switch 54 need not be connected to the key-on position.

The CSC 52 is designed to generate a continuous tempo and quantity of HCinjection when all three features (master switch position, detection ofalternator ripple and exhaust gas temperature above a pre-determinedminimum) coincide. When the CSC 52 is on, power is supplied to theinjection pump 64 and the ICU 62 that operates a solenoid valve 66 toproduce a series of pulses to enrich the exhaust gas before it passesover an oxidation catalyst upstream of the NO_(x) absorbing components.Typically the injection controller will provide occasional relativelyvery long rich pulses to ensure that the NO_(x)-trap is substantiallycompletely empty and this is followed by a more frequent sequence ofshorter enrichment pulses, e.g. injection at 2 seconds every minute, tomaintain the storing capability of the NO_(x)-trap (see FIG. 2).

This fuel injection rate is correlated to a chosen NO_(x) conversione.g. 90% at the average duty cycle speed. The exact detail of theinjection strategy depends on the vehicle and its duty cycle.

Whilst, very generally, the systems employing NO_(x)-traps describedherein have been developed to provide simple control mechanisms topredict when NO_(x)-trap regeneration should be done, with particularapplication to retrofit, many vehicles already include a range ofsensors to input data to the ECU for controlling other aspects ofvehicular operation. By suitable re-programming of the ECU it ispossible to adopt one or more of such existing sensor inputs for thepurposes of predicting remaining NO_(x)-trap capacity. These include,but are not limited to, predetermined or predicted time elapsed fromkey-on or previous regeneration, by sensing the status of a suitableclock means; airflow over the TWC or manifold vacuum; ignition timing,engine speed; throttle position; exhaust gas redox composition, forexample using a lambda sensor, preferably a linear lambda sensor,quantity of fuel injected in the engine; where the vehicle includes anexhaust gas recirculation (EGR) circuit, the position of the EGR valveand thereby the detected amount of EGR; engine coolant temperature; andwhere the exhaust system includes a NO_(x) sensor, the amount of NO_(x)detected upstream and/or downstream of the NO_(x)-trap. Where the clockembodiment is used, the predicted time can be subsequently adjusted inresponse to data input.

The following specific Example is provided by way of illustration only.

EXAMPLE

The exhaust system (10) (shown in FIG. 3) of a single deck bus fittedwith a 6 litre turbocharged engine and comprising engine turbo (12),type approved to European Stage 1 emission limits, was modified toincorporate a three-way splitter (14) for diverting the exhaust gas intoone of three parallel legs (16), the exhaust gas flow in each leg beingof equal velocity flow. Each leg (16) comprised a chamber (18)containing an oxidation catalyst (20) followed by a NO_(x)-trap (22).The gas flows were then combined downstream of the NO_(x)-traps and thetotal exhaust gas flow was passed through a “clean up” oxidationcatalyst (24) to remove any unburned hydrocarbons (HC) exiting theNO_(x)-traps before the exhaust gas was passed directly to theatmosphere. A fuel injector (26) comprising a fuel solenoid (28) wassited in front of each oxidation catalyst (20), a NO_(x) sensor (29) infront of the exhaust splitter (14), combined NO_(x)/air fuel ratiosensors (30) behind the NO_(x)-traps and thermocouples (T1, T2, T3, T4)measuring temperatures in front of and behind the oxidation catalysts(20) and at the exit of the reactors. The oxidation catalysts (20) andthe NO traps (22) were each coated on ceramic flow-through monoliths at400 cells in (62 cells cm⁻²) and 0.06 in (0.15 mm) wall thickness. Theoxidation catalysts (20) were 5.66 in (144 mm) diameter×3 in (76 mm) andvolume 75.5 in³ (1.24 litre), the NO_(x)-traps (22) were the samediameter but 6 in (152 mm) long and the “clean up” catalyst (24) 10.5 in(267 mm) diameter×3 in (76 mm) long and volume 260 in³ (4.26 litres).

The experiments described here were conducted using one leg of the splitexhaust only. The vehicle was operated using diesel fuel containing 50ppm sulphur and run at steady speeds of idle, 10, 20, 30 and 40 mph forperiods of time; fuel was injected at each of these points and the airfuel ratio during injection determined as shown in FIG. 4. Thecombination of time and duration (2 seconds injection, one per minuteper leg) was selected empirically as it gave the best combination ofexhaust gas temperatures (to maintain the NO_(x)-trap within an activetemperature window) and NO_(x) conversion. Simultaneously the NOemissions pre- and post- the system together with the temperatureprofiles were measured.

FIG. 5 shows the NO emissions (ppm) from the engine and after theNO_(x)-trap for the idle condition together with the air fuel ratiomeasured after the NO_(x)-trap. FIG. 6 shows the temperature traces forthe same period. From FIG. 5 it is seen that when fuel is injected atthe start of the idle period, the air fuel ratio drops from lean to richas expected from the predictions in FIG. 3 and, after the initial NO_(x)breakthrough, good NO_(x) conversion is seen. With time, the air fuelratio remains lean throughout the injection event but good NO_(x)conversion is still maintained. The exotherm (T2) generated over theoxidation catalyst helps maintain the temperature of the NO_(x)-trapwithin its operating window of 220-550° C. An exotherm (T3) is alsoregistered across the NO_(x)-trap, some of which is caused by combustionof unreacted gaseous reductant from the oxidation catalyst. We interpretthis result to mean that some of this exotherm is from the combustion ofunburned fuel droplets reacting on the surface of the NO_(x)-trap astime increases at this engine idle condition. This is because the systeminlet temperature falls so as to be insufficient to vaporise theincoming fuel and the rear sensor measured air/fuel ratio spikes becomeless pronounced and more rounded, suggesting a sequence of thedeposition, vaporisation and then the subsequent oxidation of the fueldroplets. The local richness caused by this event also serves tomaintain the observed NO_(x)-trap operation efficiency.

The results of the experiment with the bus held at a steady speed of 40mph are shown in FIGS. 7 and 8. Here the exhaust flow rate was muchhigher but the same injection flow rate was used as at idle and theexhaust was expected to remain lean throughout the injection periods(FIG. 3). However, apart from the breakthrough spikes when the fuel isfirst injected, NO_(x) is reduced over the remaining operating time,although not as efficiently as at idle. The exotherm (T3) over (T2) wassometimes lower than at idle, but because of the heat capacity of theincreased flow rate of exhaust gases, it is very significant. Thereforean exothermic reaction is still taking place and again we believe thatthis is because some unburned fuel droplets are being carried throughthe oxidation catalyst and being combusted on the NO_(x)-trap. Thepersistence of fuel droplets, despite the higher inlet temperature ofthe oxidation catalyst, is expected to occur because the greater exhaustflow rate that is likely to carry the droplets through the oxidationcatalyst as is shown by the significant exotherm measured across theNO_(x)-trap and the trap regeneration observed in apparently leanconditions.

FIG. 9 presents the trend in calculated average NO_(x) conversionefficiency, as a function of speed, for the system. FIG. 3 indicatesrich exhaust gas conditions do not occur above about 6 mph but goodNO_(x) conversions were obtained under lean conditions across a widerspeed range. This is especially relevant in the range from idle to 30mph which is the most common operating range for an urban city bus.

1. An exhaust system for a vehicular lean-burn internal combustionengine comprising: a NO_(x)-absorbent; a reductant injector disposedupstream of the NO_(x) absorbent; and means, when in use, forcontrolling reductant addition, wherein the reductant addition controlmeans supplies reductant to the NO_(x)-absorbent at all vehicle speedsin a duty cycle at a rate which is predetermined to correlate with adesired NO conversion at the average duty cycle speed of the vehicle. 2.An exhaust system according to claim 1, wherein the NO_(x)-absorbent isselected from the group consisting of alkaline earth metal compounds,alkali metal compounds, rare earth metal compounds and mixtures of anytwo or more thereof.
 3. An exhaust system according to claim 2, whereinthe alkaline earth metal is selected from the group consisting ofbarium, magnesium, strontium and calcium.
 4. An exhaust system accordingto claim 2, wherein the alkali metal is selected from the groupconsisting of potassium and caesium.
 5. An exhaust system according toclaim 2, wherein the rare earth metal is selected from the groupconsisting of cerium, yttrium, lanthanum and praseodymium.
 6. An exhaustsystem according to claim 2, wherein the alkaline earth metal compound,the alkali metal compound or the rare earth metal compound is supportedon a support material.
 7. An exhaust system according to claim 6,wherein the support is selected from the group consisting of alumina,silica, titania, zirconia, ceria and mixtures or a composite oxide ofany two or more thereof.
 8. An exhaust system according to claim 6,wherein the NO_(x)-absorbent comprises the support.
 9. An exhaust systemaccording to claim 1, wherein the NO_(x)-absorbent is a component of aNO_(x)-trap comprising a catalyst for oxidising NO.
 10. An exhaustsystem according to claim 9, wherein the NO oxidation catalyst comprisesa platinum group metal.
 11. An exhaust system according to claim 10,wherein the NO_(x)-trap comprises a NO reduction catalyst.
 12. Anexhaust system according to claim 1, comprising an oxidation catalystdisposed between the reductant injector and the NO_(x)-absorbent.
 13. Anexhaust system according to claim 1, comprising a catalyst for oxidisingNO to NO₂ disposed upstream of the reductant injector.
 14. An exhaustsystem according to claim 13, wherein the NO oxidation catalyst isplatinum on an alumina support.
 15. An exhaust system according to claim13, comprising an optionally catalysed particulate filter disposedbetween the oxidation catalyst and the reductant injector.
 16. Anexhaust system according to claim 9, wherein the NO_(x)-trap comprises aparticulate filter.
 17. An exhaust system according to claim 1,comprising control means when in use, intermittently to enrich theexhaust gas composition for regenerating the NO_(x)-absorbent.
 18. Anexhaust system according to claim 24, wherein the control means, when inuse, supplies reductant to the NO_(x)-trap only when the catalyst isactive for NO_(x) reduction.
 19. A diesel engine comprising an exhaustsystem according to claim
 1. 20. A light-duty diesel engine according toclaim
 19. 21. A method of reducing NO in the exhaust gas of a vehicularlean-burn internal combustion engine, which method comprising absorbingNO_(x) from the exhaust gas in a NO_(x)-absorbent, contacting theNO_(x)-absorbent with a reductant to regenerate the NO_(x)-absorbent, atall vehicle speeds in a duty cycle, and reducing NO_(x) to N₂, wherein arate of reductant injection correlates with a desired NO_(x) conversionat the average duty cycle speed.
 22. An exhaust system according toclaim 10, wherein the platinum group metal is selected from the groupconsisting of platinum, palladium and a combination of platinum andpalladium.
 23. An exhaust system according to claim 11, wherein theNO_(x) reduction catalyst comprises rhodium.
 24. An exhaust systemaccording to claim 9, comprising control means when in use, tointermittently enrich the exhaust gas composition for regenerating theNO_(x)-absorbent.